1. Field of the Invention
This invention relates to process and apparatus for restoring the rheological properties of drilling fluids, or muds, constituting slurries of clayey solids, especially to effect reduction of viscosity and gel strength of the mud without the addition of additives, and without significant change in the density of the mud.
Rotary drilling, as practiced in oil and gas production, requires the formation of a hole, or well bore, extending downwardly from the earth's surface to an oil or gas producing stratum. Formation of the well bore requires cutting into the earth with a rotating bit which is attached to the end of a drill string to which joints of pipe are sequentially attached as the well bore is extended downwardly. In the drilling operation, a drilling fluid, or mud, constituted generally of a mixture of weighing materials, clays, chemicals and water or oil is pumped down the drill string, to exit through jets in the drill bit at the bottom of the hole, the fluid, or mud, ascending to the surface via the annular space between the exterior wall of the drill string and the wall of the hole, or well bore. At the surface, the mud is pumped to a shale shaker for cuttings removal, and then returned to a mud pit or temporarily stored in a sump pit.
Drilling muds serve several essential functions, the most important of which are to seal off permeable formations of oil, gas, or water which may be encountered as the well is drilled through different subterranean formations, lubricate and cool the drill bit, lubricate the drill string, remove cuttings from the well and, in the event of a shutdown in the drilling operation, to hold the cuttings, sand and other residual materials in suspension within the static column of drilling mud. The drilling mud contains various materials and weighing agents, including particularly sustantial quantities of clays and other colloidal materials which assist in imparting the required viscosity and gel strength to the mud as required for the entrainment and suspension of the cuttings. The mud also must provide adequate lubrication of the drill bit and drill string, and it must be sufficiently fluid that it can be pumped. The gel characteristics of the mud must be sufficient to seal the formation and suppress fluid losses to the surrounding strata and, by its hydrostatic pressure, it must prevent the escape of gas or oil to prevent blowout of the well. Whereas the specific gravity or density of the mud can be readily increased by the addition of weighing materials, the drilling mud must have the proper viscosity. It must thus be thick enough to carry the cuttings and sand from the hole and suspend the cuttings and sand in the event of a shutdown, but it must also be thin enough to permit ready pumping of the mud, while yet allowing settlement and separation of the cuttings from the mud in the surface mud pits.
An ideal drilling fluid is thixotropic. A mud of this character is one whose apparent viscosity decreases as the degree of agitation or shear rate increases (as is caused by pumping or circulating the mud through the drill string); but when such agitation ceases, the mud gels sufficiently to support the cuttings and sand to prevent their resettlement back into the bottom of the well bore. Clay-based muds having thixotropic properties are used to provide the necessary particle-carrying capacity, viscosities and gel strengths. These muds thus have the property of providing a relatively high viscosity when at rest, but the viscosity is reduced when the mud is subjected to shear. This permits the mud to entrain and suspend the cuttings, sand and various other materials when the mud is at rest, or subjected to slight shear. At the same time, when the mud is subjected to shearing forces as in pumping, the viscosity is reduced sufficiently to facilitate pumping; and, on encounter with the high shearing forces at the drill bit, the viscosity of the mud is reduced sufficiently to provide improved lubrication of the bit. The requisite thixotropy, notably gel strength and viscosity, is supplied by the addition of hydratable clay or colloidal clay bodies such as bentonite or fullers earth to the mud. Over a finite period of time the clay can become hydrated to increase the viscosity and gel strength of the mud. After complete hydration of the clay constituents of a mud, additional water decreases the viscosity and gel strength of the mud.
The rheological, or flow properties, of a mud invariably change during use, especially viscosity and gel strength. This is due to the nature of the clays, e.g., bentonite, which are readily hydrated during use and which, when hydrated up to the point of maximum hydration of the clay constituents, the viscosity and gel strength of the mud is increased. Generally, the clay constituent, or constituents, of a mud gradually absorb or adsorb water and the viscosity and gel strength of the mud is increased. The acceptable range of viscosity and gel strength which a mud can possess, however, is limited, and it cannot be permitted to become too thin or too thick. When a mud becomes too thick, it must be thinned and brought back into an acceptable range of viscosity and gel strength.
During drilling operations, various conditions often cause undesired changes in the thixotropic properties of a mud, this making it necessary to constantly treat the mud to maintain the desired thixotropy, viscosity and gel strength. For example, certain formations such as encountered in the Gulf Coast areas of the United States and other areas of the world contain considerable concentrations of mud-making clays or minerals, such as montmorillonite, which tend to swell on hydration or absorption of water from the drilling mud, this increasing the viscosity and gel strength of the drilling mud.
These changes in the thixotropic properties of muds, notably increased viscosity and gel strength, necessitate continued costly reformulations of the drilling mud, especially constant thinning which calls for the addition of additives to the mud. Such additives, while serving to control a specific property as all too well known, often produce undesirable side effects as relates to other physical and chemical properties of the mud. Reformulations thus not only require constant quality control, which is costly, but additions to the mud all too soon adversely affect the overall properties of this mud so that it must be replaced. Replacement, in itself, is not only costly but creates disposal problems which, under recent EPA regulations, can in itself prove troublesome.
It is accordingly the prime objective of the present invention to provide a novel process, and apparatus, for restoring the rheological properties of a drilling mud, especially by providing means for reducing the viscosity and gel strength of such muds without the use of additives.
A more specific object is to provide process and apparatus useful at drilling sites for providing a rapid reduction of viscosity and gel strength of a thixotropic mud without change of its chemical composition, and without significant change in the density of the mud.
These and other objects are accomplished in accordance with the present invention which generally comprises:
(i) a process for reducing the viscosity and gel strength of a hydratable clay-containing drilling fluid, or mud, by contacting said drilling fluid, or mud, with an inclined surface revolving, or otherwise moving, at a rate sufficient to impart adequate compression or sheer force, or both compression and sheer forces, to dewater the hydrated clay constituent, or constituents, of said drilling fluid, or mud; and
(ii) an apparatus, or inertial device, preferably one constituted generally of structure inclusive of a revolvable cone within the inner surface of which a stream or spray of said hydratable clay-containing drilling fluid, or mud, can be impinged or contacted, when the cone is revolved at sufficient speed, to impart adequate compression or sheer force, or both compression and sheer forces, to dewater the hydrated clay constituent, or constituents, thereby reducing the viscosity and gel strength of the drilling fluid, or mud.
It has been found that the viscosity and gel strength of a drilling fluid, or mud, which contains a hydratable clay constituent, or constituents, which has been rendered highly viscous and of high gel strength by adsorption or absorption of water by the hydratable clay can be drastically reduced by impinging or contacting a stream, or jet, of said drilling fluid, or mud, upon a moving, inclined surface. In its preferred aspects, the inclined surface is cone shaped, the smaller end of the cone is closed, the larger end of the cone is open, and the cone is rotated at speed sufficient to impart an adequate compression force to dewater the hydrated clay-containing constituent, or constituents, of the mud. In a more preferred aspect, the large end of the cone is also partially closed by a non-moving wall, or baffle, which exerts an added sheer force on the mud which is expressed from the large open end, or mouth, of the revolving cone.
The small diameter end of the cone can be of full cone shape, or of frusto-conic shape, but is generally the latter and ranges from about 1 to about 3 inches in diameter. The large open end of the cone can be of virtually any diameter, but generally ranges up to about 18 inches in diameter, with preferred diameters ranging from about 9 inches to about 15 inches. The cone can be rotated at virtually any velocity sufficient to impart the compression force required to dewater the hydrated clay constituent, or constituents, of a mud. Practical speeds of revolution of the cone generally range from about 400 rpm (revolutions per minute) to about 6000 rpm for cones whose largest diameter ranges to about 18 inches. Since the compression force exerted on a hydrated clay particle injected into the small end of a cone is increased as the particle moves toward the open large diameter side of a cone it follows, of course, that the smaller diameter cones require higher rates of revolution than the larger diameter cones. Preferably, a combination of cone size and cone speed is employed to accelerate a hydrated clay particle at the small end of the cone at a rate of about 35 feet per second, each second, up to about 500 feet per second, each second, and more preferably from about 35 feet per second, each second, to about 180 feet per second, each second, at the large open end, or mouth, of the cone. Such particle acceleration can be imparted, e.g., by use of a frusto-conic shaped cone having a diameter at its smaller closed end of 21/4 inches and a diameter at its larger open end of 113/4 inches by rotating the cone at a speed of 3500 rpm.
In use, the stream of drilling fluid, or mud, is injected directly into the bottom or closed end of the cone, preferably in a path coincident or parallel with the central axis through the center of the cone. The drilling fluid, or mud, is projected or thrown against the rotating inclined surface, the surface of the cone forming an angle of inclination ranging from about 20.degree. to about 60.degree., preferably from about 30.degree. to about 45.degree., as measured by the angle formed between the central axis of the cone, or the path of the projected drilling fluid, or mud, and the rotating surface. The surface of the cone is rotated at linear velocities ranging, at the large diameter side of the cone about 180 feet per second, and higher, preferably from about 180 to about 500 feet per second which is sufficient to adequately dewater most clays and lower the viscosity and gel strength of the mud by producing substantial separation of water from the particulate hydrated clays on contact of the drilling fluid, or mud, with the moving, inclined surface. On separation of the water from the clay, the drilling fluid, or mud, is constituted of an admixture which includes gel, solids particles and water, or such admixture which obtains gel, solids particles and an increased amount of water released from the clay of the drilling fluid, or mud, whose viscosity and gel strength are reduced. Rehydration of the clay with water can be suppressed by continuous recirculation of the mud through the cone, the time between contacts being frequent enough not to allow time for rehydration; or, an additive, e.g., lignosulfonate or another type of thinner, can be added to suppress rehydration of the clay.
In a preferred apparatus embodiment, a cone is mounted with its smaller end closed on a revolvable shaft, and an inlet through which a stream of a hydratable clay-containing drilling fluid, or mud, can be directed into the large open end, or mouth, of the rotating cone such that, when the cone is rotated at sufficient angular velocity, the hydrated clay of the drilling fluid, or mud, is dehydrated by compression and an admixture comprising gel, solid particles and water is expressed from the edges around the mouth of the cone, and the viscosity and gel strength of the expressed drilling fluid, or mud, is drastically reduced as contrasted with the drilling fluid, or mud, prior to such treatment. Preferably, the cone provides an inner surface having an angle of inclination from a central axis through the center thereof ranging from about 20.degree. to about 60.degree., preferably from about 30.degree. to about 45.degree.. This means, in effect, that the total included angle formed by opposed surfaces within the mouth of the cone ranges from about 40.degree. to about 120.degree., preferably from about 60.degree. to about 90.degree.. The cone is generally revolved at speeds ranging from about 400 to about 6000 rpm, preferably from about 2500 to about 4500 rpm, such speeds generally being sufficient to inertially separate the water from the hydrated clays of the drilling fluid, or mud, thereby reducing its viscosity and gel strength.
Having reduced the viscosity and gel strength, gel reformation can be avoided by various known methods. Whereas the separation of water from the hydrated clays can be effected without any need for additives, some selected additives can be added, if desired, to directly inhibit or suppress reformation of the gel, or combine with the excess water to prevent reformation of the gel. Or, on the other hand, the water can be recirculated and again contacted with the cone, or physically separated from the admixture before the clay and water have had time to reform a gel.
The invention, including both the process and apparatus, and the principle of operation,, will be more fully understood by reference to the following detailed description of a preferred embodiment, and to the attached drawing to which reference is made as the description unfolds. Similar numbers are used to represent similar parts or components in the figure.